Attention-based sequence transduction neural networks

ABSTRACT

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for generating an output sequence from an input sequence. In one aspect, one of the systems includes an encoder neural network configured to receive the input sequence and generate encoded representations of the network inputs, the encoder neural network comprising a sequence of one or more encoder subnetworks, each encoder subnetwork configured to receive a respective encoder subnetwork input for each of the input positions and to generate a respective subnetwork output for each of the input positions, and each encoder subnetwork comprising: an encoder self-attention sub-layer that is configured to receive the subnetwork input for each of the input positions and, for each particular input position in the input order: apply an attention mechanism over the encoder subnetwork inputs using one or more queries derived from the encoder subnetwork input at the particular input position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 16/932,422, filed on Jul. 17, 2020, which is acontinuation of and claims priority to U.S. application Ser. No.16/559,392, filed on Sep. 3, 2019 (now U.S. Pat. No. 10,719,764), whichis a continuation of and claims priority to U.S. application Ser. No.16/021,971, filed on Jun. 28, 2018 (now U.S. Pat. No. 10,452,978), whichis a continuation of and claims priority to PCT Application No.PCT/US2018/034224, filed on May 23, 2018, which claims priority to U.S.Provisional Application No. 62/510,256, filed on May 23, 2017, and U.S.Provisional Application No. 62/541,594, filed on Aug. 4, 2017. Theentire contents of the foregoing applications are hereby incorporated byreference.

BACKGROUND

This specification relates to transducing sequences using neuralnetworks.

Neural networks are machine learning models that employ one or morelayers of nonlinear units to predict an output for a received input.Some neural networks include one or more hidden layers in addition to anoutput layer. The output of each hidden layer is used as input to thenext layer in the network, i.e., the next hidden layer or the outputlayer. Each layer of the network generates an output from a receivedinput in accordance with current values of a respective set ofparameters.

SUMMARY

This specification describes a system implemented as computer programson one or more computers in one or more locations that generates anoutput sequence that includes a respective output at each of multiplepositions in an output order from an input sequence that includes arespective input at each of multiple positions in an input order, i.e.,transduces the input sequence into the output sequence. In particular,the system generates the output sequence using an encoder neural networkand a decoder neural network that are both attention-based.

Particular embodiments of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages.

Many existing approaches to sequence transduction using neural networksuse recurrent neural networks in both the encoder and the decoder. Whilethese kinds of networks can achieve good performance on sequencetransduction tasks, their computation is sequential in nature, i.e., arecurrent neural network generates an output at a current time stepconditioned on the hidden state of the recurrent neural network at thepreceding time step. This sequential nature precludes parallelization,resulting in long training and inference times and, accordingly,workloads that utilize a large amount of computational resources.

On the other hand, because the encoder and the decoder of the describedsequence transduction neural network are attention-based, the sequencetransduction neural network can transduce sequences quicker, be trainedfaster, or both, because the operation of the network can be more easilyparallelized. That is, because the described sequence transductionneural network relies entirely on an attention mechanism to draw globaldependencies between input and output and does not employ any recurrentneural network layers, the problems with long training and inferencetimes and high resource usage caused by the sequential nature ofrecurrent neural network layers are mitigated.

Moreover, the sequence transduction neural network can transducesequences more accurately than existing networks that are based onconvolutional layers or recurrent layers, even though training andinference times are shorter. In particular, in conventional models, thenumber of operations required to relate signals from two arbitrary inputor output positions grows with the distance between positions, e.g.,either linearly or logarithmically depending on the model architecture.This makes it more difficult to learn dependencies between distantpositions during training. In the presently described sequencetransduction neural network, this number of operations is reduced to aconstant number of operations because of the use of attention (and, inparticular, self-attention) while not relying on recurrence orconvolutions. Self-attention, sometimes called intra-attention, is anattention mechanism relating different positions of a single sequence inorder to compute a representation of the sequence. The use of attentionmechanisms allows the sequence transduction neural network toeffectively learn dependencies between distant positions duringtraining, improving the accuracy of the sequence transduction neuralnetwork on various transduction tasks, e.g., machine translation. Infact, the described sequence transduction neural network can achievestate-of-the-art results on the machine translation task despite beingeasier to train and quicker to generate outputs than conventionalmachine translation neural networks. The sequence transduction neuralnetwork can also exhibit improved performance over conventional machinetranslation neural networks without task-specific tuning through the useof the attention mechanism.

The details of one or more embodiments of the subject matter of thisspecification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example neural network system.

FIG. 2 is a diagram showing attention mechanisms that are applied by theattention sub-layers in the subnetworks of the encoder neural networkand the decoder neural network.

FIG. 3 is a flow diagram of an example process for generating an outputsequence from an input sequence.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

This specification describes a system implemented as computer programson one or more computers in one or more locations that generates anoutput sequence that includes a respective output at each of multiplepositions in an output order from an input sequence that includes arespective input at each of multiple positions in an input order, i.e.,transduces the input sequence into the output sequence.

For example, the system may be a neural machine translation system. Thatis, if the input sequence is a sequence of words in an originallanguage, e.g., a sentence or phrase, the output sequence may be atranslation of the input sequence into a target language, i.e., asequence of words in the target language that represents the sequence ofwords in the original language.

As another example, the system may be a speech recognition system. Thatis, if the input sequence is a sequence of audio data representing aspoken utterance, the output sequence may be a sequence of graphemes,characters, or words that represents the utterance, i.e., is atranscription of the input sequence.

As another example, the system may be a natural language processingsystem. For example, if the input sequence is a sequence of words in anoriginal language, e.g., a sentence or phrase, the output sequence maybe a summary of the input sequence in the original language, i.e., asequence that has fewer words than the input sequence but that retainsthe essential meaning of the input sequence. As another example, if theinput sequence is a sequence of words that form a question, the outputsequence can be a sequence of words that form an answer to the question.

As another example, the system may be part of a computer-assistedmedical diagnosis system. For example, the input sequence can be asequence of data from an electronic medical record and the outputsequence can be a sequence of predicted treatments.

As another example, the system may be part of an image processingsystem. For example, the input sequence can be an image, i.e., asequence of color values from the image, and the output can be asequence of text that describes the image. As another example, the inputsequence can be a sequence of text or a different context and the outputsequence can be an image that describes the context.

In particular, the neural network includes an encoder neural network anda decoder neural network. Generally, both the encoder and the decoderare attention-based, i.e., both apply an attention mechanism over theirrespective received inputs while transducing the input sequence. In somecases, neither the encoder nor the decoder include any convolutionallayers or any recurrent layers.

FIG. 1 shows an example neural network system 100. The neural networksystem 100 is an example of a system implemented as computer programs onone or more computers in one or more locations, in which the systems,components, and techniques described below can be implemented.

The neural network system 100 receives an input sequence 102 andprocesses the input sequence 102 to transduce the input sequence 102into an output sequence 152.

The input sequence 102 has a respective network input at each ofmultiple input positions in an input order and the output sequence 152has a respective network output at each of multiple output positions inan output order. That is, the input sequence 102 has multiple inputsarranged according to an input order and the output sequence 152 hasmultiple outputs arranged according to an output order.

As described above, the neural network system 100 can perform any of avariety of tasks that require processing sequential inputs to generatesequential outputs.

The neural network system 100 includes an attention-based sequencetransduction neural network 108, which in turn includes an encoderneural network 110 and a decoder neural network 150.

The encoder neural network 110 is configured to receive the inputsequence 102 and generate a respective encoded representation of each ofthe network inputs in the input sequence. Generally, an encodedrepresentation is a vector or other ordered collection of numericvalues.

The decoder neural network 150 is then configured to use the encodedrepresentations of the network inputs to generate the output sequence152.

Generally, and as will be described in more detail below, both theencoder 110 and the decoder 150 are attention-based. In some cases,neither the encoder nor the decoder include any convolutional layers orany recurrent layers.

The encoder neural network 110 includes an embedding layer 120 and asequence of one or more encoder subnetworks 130. In particular, as shownin FIG. 1, the encoder neural network includes N encoder subnetworks130.

The embedding layer 120 is configured to, for each network input in theinput sequence, map the network input to a numeric representation of thenetwork input in an embedding space, e.g., into a vector in theembedding space. The embedding layer 120 then provides the numericrepresentations of the network inputs to the first subnetwork in thesequence of encoder subnetworks 130, i.e., to the first encodersubnetwork 130 of the N encoder subnetworks 130.

In particular, in some implementations, the embedding layer 120 isconfigured to map each network input to an embedded representation ofthe network input and then combine, e.g., sum or average, the embeddedrepresentation of the network input with a positional embedding of theinput position of the network input in the input order to generate acombined embedded representation of the network input. That is, eachposition in the input sequence has a corresponding embedding and foreach network input the embedding layer 120 combines the embeddedrepresentation of the network input with the embedding of the networkinput's position in the input sequence. Such positional embeddings canenable the model to make full use of the order of the input sequencewithout relying on recurrence or convolutions.

In some cases, the positional embeddings are learned. As used in thisspecification, the term “learned” means that an operation or a value hasbeen adjusted during the training of the sequence transduction neuralnetwork 108. Training the sequence transduction neural network 108 isdescribed below with reference to FIG. 3. In some other cases, thepositional embeddings are fixed and are different for each position. Forexample, the embeddings can be made up of sine and cosine functions ofdifferent frequencies and can satisfy:

PE_((pos,2i))=sin(pos/1000^(2i/d) ^(model) ).

PE_((pos,2i+1))=cos(pos/1000^(2i/d) ^(model) ),

where pos is the position, i is the dimension within the positionalembedding, and d_(model) is the dimensionality of the positionalembedding (and of the other vectors processed by the neural network108). The use of sinusoidal positional embeddings may allow the model toextrapolate to longer sequence lengths, which can increase the range ofapplications for which the model can be employed.

The combined embedded representation is then used as the numericrepresentation of the network input.

Each of the encoder subnetworks 130 is configured to receive arespective encoder subnetwork input for each of the plurality of inputpositions and to generate a respective subnetwork output for each of theplurality of input positions.

The encoder subnetwork outputs generated by the last encoder subnetworkin the sequence are then used as the encoded representations of thenetwork inputs.

For the first encoder subnetwork in the sequence, the encoder subnetworkinput is the numeric representations generated by the embedding layer120, and, for each encoder subnetwork other than the first encodersubnetwork in the sequence, the encoder subnetwork input is the encodersubnetwork output of the preceding encoder subnetwork in the sequence.

Each encoder subnetwork 130 includes an encoder self-attention sub-layer132. The encoder self-attention sub-layer 132 is configured to receivethe subnetwork input for each of the plurality of input positions and,for each particular input position in the input order, apply anattention mechanism over the encoder subnetwork inputs at the inputpositions using one or more queries derived from the encoder subnetworkinput at the particular input position to generate a respective outputfor the particular input position. In some cases, the attentionmechanism is a multi-head attention mechanism. The attention mechanismand how the attention mechanism is applied by the encoder self-attentionsub-layer 132 will be described in more detail below with reference toFIG. 2.

In some implementations, each of the encoder subnetworks 130 alsoincludes a residual connection layer that combines the outputs of theencoder self-attention sub-layer with the inputs to the encoderself-attention sub-layer to generate an encoder self-attention residualoutput and a layer normalization layer that applies layer normalizationto the encoder self-attention residual output. These two layers arecollectively referred to as an “Add & Norm” operation in FIG. 1.

Some or all of the encoder subnetworks can also include a position-wisefeed-forward layer 134 that is configured to operate on each position inthe input sequence separately. In particular, for each input position,the feed-forward layer 134 is configured receive an input at the inputposition and apply a sequence of transformations to the input at theinput position to generate an output for the input position. Forexample, the sequence of transformations can include two or more learnedlinear transformations each separated by an activation function, e.g., anon-linear elementwise activation function, e.g., a ReLU activationfunction, which can allow for faster and more effective training onlarge and complex datasets. The inputs received by the position-wisefeed-forward layer 134 can be the outputs of the layer normalizationlayer when the residual and layer normalization layers are included orthe outputs of the encoder self-attention sub-layer 132 when theresidual and layer normalization layers are not included. Thetransformations applied by the layer 134 will generally be the same foreach input position (but different feed-forward layers in differentsubnetworks will apply different transformations).

In cases where an encoder subnetwork 130 includes a position-wisefeed-forward layer 134, the encoder subnetwork can also include aresidual connection layer that combines the outputs of the position-wisefeed-forward layer with the inputs to the position-wise feed-forwardlayer to generate an encoder position-wise residual output and a layernormalization layer that applies layer normalization to the encoderposition-wise residual output. These two layers are also collectivelyreferred to as an “Add & Norm” operation in FIG. 1. The outputs of thislayer normalization layer can then be used as the outputs of the encodersubnetwork 130.

Once the encoder neural network 110 has generated the encodedrepresentations, the decoder neural network 150 is configured togenerate the output sequence in an auto-regressive manner.

That is, the decoder neural network 150 generates the output sequence,by at each of a plurality of generation time steps, generating a networkoutput for a corresponding output position conditioned on (i) theencoded representations and (ii) network outputs at output positionspreceding the output position in the output order.

In particular, for a given output position, the decoder neural networkgenerates an output that defines a probability distribution overpossible network outputs at the given output position. The decoderneural network can then select a network output for the output positionby sampling from the probability distribution or by selecting thenetwork output with the highest probability.

Because the decoder neural network 150 is auto-regressive, at eachgeneration time step, the decoder 150 operates on the network outputsthat have already been generated before the generation time step, i.e.,the network outputs at output positions preceding the correspondingoutput position in the output order. In some implementations, to ensurethis is the case during both inference and training, at each generationtime step the decoder neural network 150 shifts the already generatednetwork outputs right by one output order position (i.e., introduces aone position offset into the already generated network output sequence)and (as will be described in more detail below) masks certain operationsso that positions can only attend to positions up to and including thatposition in the output sequence (and not subsequent positions). Whilethe remainder of the description below describes that, when generating agiven output at a given output position, various components of thedecoder 150 operate on data at output positions preceding the givenoutput positions (and not on data at any other output positions), itwill be understood that this type of conditioning can be effectivelyimplemented using the shifting described above.

The decoder neural network 150 includes an embedding layer 160, asequence of decoder subnetworks 170, a linear layer 180, and a softmaxlayer 190. In particular, as shown in FIG. 1, the decoder neural networkincludes N decoder subnetworks 170. However, while the example of FIG. 1shows the encoder 110 and the decoder 150 including the same number ofsubnetworks, in some cases the encoder 110 and the decoder 150 includedifferent numbers of subnetworks. That is, the decoder 150 can includemore or fewer subnetworks than the encoder 110.

The embedding layer 160 is configured to, at each generation time step,for each network output at an output position that precedes the currentoutput position in the output order, map the network output to a numericrepresentation of the network output in the embedding space. Theembedding layer 160 then provides the numeric representations of thenetwork outputs to the first subnetwork 170 in the sequence of decodersubnetworks, i.e., to the first decoder subnetwork 170 of the N decodersubnetworks.

In particular, in some implementations, the embedding layer 160 isconfigured to map each network output to an embedded representation ofthe network output and combine the embedded representation of thenetwork output with a positional embedding of the output position of thenetwork output in the output order to generate a combined embeddedrepresentation of the network output. The combined embeddedrepresentation is then used as the numeric representation of the networkoutput. The embedding layer 160 generates the combined embeddedrepresentation in the same manner as described above with reference tothe embedding layer 120.

Each decoder subnetwork 170 is configured to, at each generation timestep, receive a respective decoder subnetwork input for each of theplurality of output positions preceding the corresponding outputposition and to generate a respective decoder subnetwork output for eachof the plurality of output positions preceding the corresponding outputposition (or equivalently, when the output sequence has been shiftedright, each network output at a position up to and including the currentoutput position).

In particular, each decoder subnetwork 170 includes two differentattention sub-layers: a decoder self-attention sub-layer 172 and anencoder-decoder attention sub-layer 174.

Each decoder self-attention sub-layer 172 is configured to, at eachgeneration time step, receive an input for each output positionpreceding the corresponding output position and, for each of theparticular output positions, apply an attention mechanism over theinputs at the output positions preceding the corresponding positionusing one or more queries derived from the input at the particularoutput position to generate a updated representation for the particularoutput position. That is, the decoder self-attention sub-layer 172applies an attention mechanism that is masked so that it does not attendover or otherwise process any data that is not at a position precedingthe current output position in the output sequence.

Each encoder-decoder attention sub-layer 174, on the other hand, isconfigured to, at each generation time step, receive an input for eachoutput position preceding the corresponding output position and, foreach of the output positions, apply an attention mechanism over theencoded representations at the input positions using one or more queriesderived from the input for the output position to generate an updatedrepresentation for the output position. Thus, the encoder-decoderattention sub-layer 174 applies attention over encoded representationswhile the encoder self-attention sub-layer 172 applies attention overinputs at output positions.

The attention mechanism applied by each of these attention sub-layerswill be described in more detail below with reference to FIG. 2.

In FIG. 1, the decoder self-attention sub-layer 172 is shown as beingbefore the encoder-decoder attention sub-layer in the processing orderwithin the decoder subnetwork 170. In other examples, however, thedecoder self-attention sub-layer 172 may be after the encoder-decoderattention sub-layer 174 in the processing order within the decodersubnetwork 170 or different subnetworks may have different processingorders.

In some implementations, each decoder subnetwork 170 includes, after thedecoder self-attention sub-layer 172, after the encoder-decoderattention sub-layer 174, or after each of the two sub-layers, a residualconnection layer that combines the outputs of the attention sub-layerwith the inputs to the attention sub-layer to generate a residual outputand a layer normalization layer that applies layer normalization to theresidual output. FIG. 1 shows these two layers being inserted after eachof the two sub-layers, both referred to as an “Add & Norm” operation.

Some or all of the decoder subnetwork 170 also include a position-wisefeed-forward layer 176 that is configured to operate in a similar manneras the position-wise feed-forward layer 134 from the encoder 110. Inparticular, the layer 176 is configured to, at each generation timestep: for each output position preceding the corresponding outputposition: receive an input at the output position, and apply a sequenceof transformations to the input at the output position to generate anoutput for the output position. For example, the sequence oftransformations can include two or more learned linear transformationseach separated by an activation function, e.g., a non-linear elementwiseactivation function, e.g., a ReLU activation function. The inputsreceived by the position-wise feed-forward layer 176 can be the outputsof the layer normalization layer (following the last attention sub-layerin the subnetwork 170) when the residual and layer normalization layersare included or the outputs of the last attention sub-layer in thesubnetwork 170 when the residual and layer normalization layers are notincluded.

In cases where a decoder subnetwork 170 includes a position-wisefeed-forward layer 176, the decoder subnetwork can also include aresidual connection layer that combines the outputs of the position-wisefeed-forward layer with the inputs to the position-wise feed-forwardlayer to generate a decoder position-wise residual output and a layernormalization layer that applies layer normalization to the decoderposition-wise residual output. These two layers are also collectivelyreferred to as an “Add & Norm” operation in FIG. 1. The outputs of thislayer normalization layer can then be used as the outputs of the decodersubnetwork 170.

At each generation time step, the linear layer 180 applies a learnedlinear transformation to the output of the last decoder subnetwork 170in order to project the output of the last decoder subnetwork 170 intothe appropriate space for processing by the softmax layer 190. Thesoftmax layer 190 then applies a softmax function over the outputs ofthe linear layer 180 to generate the probability distribution over thepossible network outputs at the generation time step. As describedabove, the decoder 150 can then select a network output from thepossible network outputs using the probability distribution.

FIG. 2 is a diagram 200 showing attention mechanisms that are applied bythe attention sub-layers in the subnetworks of the encoder neuralnetwork 110 and the decoder neural network 150.

Generally, an attention mechanism maps a query and a set of key-valuepairs to an output, where the query, keys, and values are all vectors.The output is computed as a weighted sum of the values, where the weightassigned to each value is computed by a compatibility function of thequery with the corresponding key.

More specifically, each attention sub-layer applies a scaled dot-productattention mechanism 230. In scaled dot-product attention, for a givenquery, the attention sub-layer computes the dot products of the querywith all of the keys, divides each of the dot products by a scalingfactor, e.g., by the square root of the dimensions of the queries andkeys, and then applies a softmax function over the scaled dot productsto obtain the weights on the values. The attention sub-layer thencomputes a weighted sum of the values in accordance with these weights.Thus, for scaled dot-product attention the compatibility function is thedot product and the output of the compatibility function is furtherscaled by the scaling factor.

In operation and as shown in the left hand side of FIG. 2, the attentionsub-layer computes the attention over a set of queries simultaneously.In particular, the attention sub-layer packs the queries into a matrixQ, packs the keys into a matrix K, and packs the values into a matrix V.To pack a set of vectors into a matrix, the attention sub-layer cangenerate a matrix that includes the vectors as the rows of the matrix.

The attention sub-layer then performs a matrix multiply (MatMul) betweenthe matrix Q and the transpose of the matrix K to generate a matrix ofcompatibility function outputs.

The attention sub-layer then scales the compatibility function outputmatrix, i.e., by dividing each element of the matrix by the scalingfactor.

The attention sub-layer then applies a softmax over the scaled outputmatrix to generate a matrix of weights and performs a matrix multiply(MatMul) between the weight matrix and the matrix V to generate anoutput matrix that includes the output of the attention mechanism foreach of the values.

For sub-layers that use masking, i.e., decoder attention sub-layers, theattention sub-layer masks the scaled output matrix before applying thesoftmax. That is, the attention sub-layer masks out (sets to negativeinfinity), all values in the scaled output matrix that correspond topositions after the current output position.

In some implementations, to allow the attention sub-layers to jointlyattend to information from different representation subspaces atdifferent positions, the attention sub-layers employ multi-headattention, as illustrated on the right hand side of FIG. 2.

In particular, to implement multi-ahead attention, the attentionsub-layer applies h different attention mechanisms in parallel. In otherwords, the attention sub-layer includes h different attention layers,with each attention layer within the same attention sub-layer receivingthe same original queries Q, original keys K, and original values V.

Each attention layer is configured to transform the original queries,and keys, and values using learned linear transformations and then applythe attention mechanism 230 to the transformed queries, keys, andvalues. Each attention layer will generally learn differenttransformations from each other attention layer in the same attentionsub-layer.

In particular, each attention layer is configured to apply a learnedquery linear transformation to each original query to generate alayer-specific query for each original query, apply a learned key lineartransformation to each original key to generate a layer-specific key foreach original key, and apply a learned value linear transformation toeach original value to generate a layer-specific values for eachoriginal value. The attention layer then applies the attention mechanismdescribed above using these layer-specific queries, keys, and values togenerate initial outputs for the attention layer.

The attention sub-layer then combines the initial outputs of theattention layers to generate the final output of the attentionsub-layer. As shown in FIG. 2, the attention sub-layer concatenates(concat) the outputs of the attention layers and applies a learnedlinear transformation to the concatenated output to generate the outputof the attention sub-layer.

In some cases, the learned transformations applied by the attentionsub-layer reduce the dimensionality of the original keys and values and,optionally, the queries. For example, when the dimensionality of theoriginal keys, values, and queries is d and there are h attention layersin the sub-layer, the sub-layer may reduce the dimensionality of theoriginal keys, values, and queries to d/h. This keeps the computationcost of the multi-head attention mechanism similar to what the costwould have been to perform the attention mechanism once with fulldimensionality while at the same time increasing the representativecapacity of the attention sub-layer.

While the attention mechanism applied by each attention sub-layer is thesame, the queries, keys, and values are different for different types ofattention. That is, different types of attention sub-layers usedifferent sources for the original queries, keys, and values that arereceived as input by the attention sub-layer.

In particular, when the attention sub-layer is an encoder self-attentionsub-layer, all of the keys, values and queries come from the same place,in this case, the output of the previous subnetwork in the encoder, or,for the encoder self-attention sub-layer in first subnetwork, theembeddings of the inputs and each position in the encoder can attend toall positions in the input order. Thus, there is a respective key,value, and query for each position in the input order.

When the attention sub-layer is a decoder self-attention sub-layer, eachposition in the decoder attends to all positions in the decoderpreceding that position. Thus, all of the keys, values, and queries comefrom the same place, in this case, the output of the previous subnetworkin the decoder, or, for the decoder self-attention sub-layer in thefirst decoder subnetwork, the embeddings of the outputs alreadygenerated. Thus, there is a respective key, value, and query for eachposition in the output order before the current position.

When the attention sub-layer is an encoder-decoder attention sub-layer,the queries come from the previous component in the decoder and the keysand values come from the output of the encoder, i.e., from the encodedrepresentations generated by the encoder. This allows every position inthe decoder to attend over all positions in the input sequence. Thus,there is a respective query for each for each position in the outputorder before the current position and a respective key and a respectivevalue for each position in the input order.

In more detail, when the attention sub-layer is an encoderself-attention sub-layer, for each particular input position in theinput order, the encoder self-attention sub-layer is configured to applyan attention mechanism over the encoder subnetwork inputs at the inputpositions using one or more queries derived from the encoder subnetworkinput at the particular input position to generate a respective outputfor the particular input position.

When the encoder self-attention sub-layer implements multi-headattention, each encoder self-attention layer in the encoderself-attention sub-layer is configured to: apply a learned query lineartransformation to each encoder subnetwork input at each input positionto generate a respective query for each input position, apply a learnedkey linear transformation to each encoder subnetwork input at each inputposition to generate a respective key for each input position, apply alearned value linear transformation to each encoder subnetwork input ateach input position to generate a respective value for each inputposition, and then apply the attention mechanism (i.e., the scaleddot-product attention mechanism described above) using the queries,keys, and values to determine an initial encoder self-attention outputfor each input position. The sub-layer then combines the initial outputsof the attention layers as described above.

When the attention sub-layer is a decoder self-attention sub-layer, thedecoder self-attention sub-layer is configured to, at each generationtime step: receive an input for each output position preceding thecorresponding output position and, for each of the particular outputpositions, apply an attention mechanism over the inputs at the outputpositions preceding the corresponding position using one or more queriesderived from the input at the particular output position to generate aupdated representation for the particular output position.

When the decoder self-attention sub-layer implements multi-headattention, each attention layer in the decoder self-attention sub-layeris configured to, at each generation time step, apply a learned querylinear transformation to the input at each output position preceding thecorresponding output position to generate a respective query for eachoutput position, apply a learned key linear transformation to each inputat each output position preceding the corresponding output position togenerate a respective key for each output position, apply a learnedvalue linear transformation to each input at each output positionpreceding the corresponding output position to generate a respective keyfor each output position, and then apply the attention mechanism (i.e.,the scaled dot-product attention mechanism described above) using thequeries, keys, and values to determine an initial decoder self-attentionoutput for each of the output positions. The sub-layer then combines theinitial outputs of the attention layers as described above.

When the attention sub-layer is an encoder-decoder attention sub-layer,the encoder-decoder attention sub-layer is configured to, at eachgeneration time step: receive an input for each output positionpreceding the corresponding output position and, for each of the outputpositions, apply an attention mechanism over the encoded representationsat the input positions using one or more queries derived from the inputfor the output position to generate an updated representation for theoutput position.

When the encoder-decoder attention sub-layer implements multi-headattention, each attention layer is configured to, at each generationtime step: apply a learned query linear transformation to the input ateach output position preceding the corresponding output position togenerate a respective query for each output position, apply a learnedkey linear transformation to each encoded representation at each inputposition to generate a respective key for each input position, apply alearned value linear transformation to each encoded representation ateach input position to generate a respective value for each inputposition, and then apply the attention mechanism (i.e., the scaleddot-product attention mechanism described above) using the queries,keys, and values to determine an initial encoder-decoder attentionoutput for each input position. The sub-layer then combines the initialoutputs of the attention layers as described above.

FIG. 3 is a flow diagram of an example process for generating an outputsequence from an input sequence. For convenience, the process 300 willbe described as being performed by a system of one or more computerslocated in one or more locations. For example, a neural network system,e.g., neural network system 100 of FIG. 1, appropriately programmed inaccordance with this specification, can perform the process 300.

The system receives an input sequence (step 310).

The system processes the input sequence using the encoder neural networkto generate a respective encoded representation of each of the networkinputs in the input sequence (step 320). In particular, the systemprocesses the input sequence through the embedding layer to generate anembedded representation of each network input and then process theembedded representations through the sequence of encoder subnetworks togenerate the encoded representations of the network inputs.

The system processes the encoded representations using the decoderneural network to generate an output sequence (step 330). The decoderneural network is configured to generate the output sequence from theencoded representations in an auto-regressive manner. That is, thedecoder neural network generates one output from the output sequence ateach generation time step. At a given generation time step at which agiven output is being generated, the system processes the outputs beforethe given output in the output sequence through the embedding layer inthe decoder to generate embedded representations. The system thenprocesses the embedded representations through the sequence of decodersubnetworks, the linear layer, and the softmax layer to generate thegiven output. Because the decoder subnetworks include encoder-decoderattention sub-layers as well as decoder self-attention sub-layers, thedecoder makes use of both the already generated outputs and the encodedrepresentations when generating the given output.

The system can perform the process 300 for input sequences for which thedesired output, i.e., the output sequence that should be generated bythe system for the input sequence, is not known.

The system can also perform the process 300 on input sequences in a setof training data, i.e., a set of inputs for which the output sequencethat should be generated by the system is known, in order to train theencoder and the decoder to determine trained values for the parametersof the encoder and decoder. The process 300 can be performed repeatedlyon inputs selected from a set of training data as part of a conventionalmachine learning training technique to train the initial neural networklayers, e.g., a gradient descent with backpropagation training techniquethat uses a conventional optimizer, e.g., the Adam optimizer. Duringtraining, the system can incorporate any number of techniques to improvethe speed, the effectiveness, or both of the training process. Forexample, the system can use dropout, label smoothing, or both to reduceoverfitting. As another example, the system can perform the trainingusing a distributed architecture that trains multiple instances of thesequence transduction neural network in parallel.

This specification uses the term “configured” in connection with systemsand computer program components. For a system of one or more computersto be configured to perform particular operations or actions means thatthe system has installed on it software, firmware, hardware, or acombination of them that in operation cause the system to perform theoperations or actions. For one or more computer programs to beconfigured to perform particular operations or actions means that theone or more programs include instructions that, when executed by dataprocessing apparatus, cause the apparatus to perform the operations oractions.

Embodiments of the subject matter and the functional operationsdescribed in this specification can be implemented in digital electroniccircuitry, in tangibly-embodied computer software or firmware, incomputer hardware, including the structures disclosed in thisspecification and their structural equivalents, or in combinations ofone or more of them. Embodiments of the subject matter described in thisspecification can be implemented as one or more computer programs, i.e.,one or more modules of computer program instructions encoded on atangible non transitory storage medium for execution by, or to controlthe operation of, data processing apparatus. The computer storage mediumcan be a machine-readable storage device, a machine-readable storagesubstrate, a random or serial access memory device, or a combination ofone or more of them. Alternatively or in addition, the programinstructions can be encoded on an artificially generated propagatedsignal, e.g., a machine-generated electrical, optical, orelectromagnetic signal, that is generated to encode information fortransmission to suitable receiver apparatus for execution by a dataprocessing apparatus.

The term “data processing apparatus” refers to data processing hardwareand encompasses all kinds of apparatus, devices, and machines forprocessing data, including by way of example a programmable processor, acomputer, or multiple processors or computers. The apparatus can alsobe, or further include, special purpose logic circuitry, e.g., an FPGA(field programmable gate array) or an ASIC (application specificintegrated circuit). The apparatus can optionally include, in additionto hardware, code that creates an execution environment for computerprograms, e.g., code that constitutes processor firmware, a protocolstack, a database management system, an operating system, or acombination of one or more of them.

A computer program, which may also be referred to or described as aprogram, software, a software application, an app, a module, a softwaremodule, a script, or code, can be written in any form of programminglanguage, including compiled or interpreted languages, or declarative orprocedural languages; and it can be deployed in any form, including as astand alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment. A program may, but neednot, correspond to a file in a file system. A program can be stored in aportion of a file that holds other programs or data, e.g., one or morescripts stored in a markup language document, in a single file dedicatedto the program in question, or in multiple coordinated files, e.g.,files that store one or more modules, sub programs, or portions of code.A computer program can be deployed to be executed on one computer or onmultiple computers that are located at one site or distributed acrossmultiple sites and interconnected by a data communication network.

In this specification, the term “database” is used broadly to refer toany collection of data: the data does not need to be structured in anyparticular way, or structured at all, and it can be stored on storagedevices in one or more locations. Thus, for example, the index databasecan include multiple collections of data, each of which may be organizedand accessed differently.

Similarly, in this specification the term “engine” is used broadly torefer to a software-based system, subsystem, or process that isprogrammed to perform one or more specific functions. Generally, anengine will be implemented as one or more software modules orcomponents, installed on one or more computers in one or more locations.In some cases, one or more computers will be dedicated to a particularengine; in other cases, multiple engines can be installed and running onthe same computer or computers.

The processes and logic flows described in this specification can beperformed by one or more programmable computers executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby special purpose logic circuitry, e.g., an FPGA or an ASIC, or by acombination of special purpose logic circuitry and one or moreprogrammed computers.

Computers suitable for the execution of a computer program can be basedon general or special purpose microprocessors or both, or any other kindof central processing unit. Generally, a central processing unit willreceive instructions and data from a read only memory or a random accessmemory or both. The essential elements of a computer are a centralprocessing unit for performing or executing instructions and one or morememory devices for storing instructions and data. The central processingunit and the memory can be supplemented by, or incorporated in, specialpurpose logic circuitry. Generally, a computer will also include, or beoperatively coupled to receive data from or transfer data to, or both,one or more mass storage devices for storing data, e.g., magnetic,magneto optical disks, or optical disks. However, a computer need nothave such devices. Moreover, a computer can be embedded in anotherdevice, e.g., a mobile telephone, a personal digital assistant (PDA), amobile audio or video player, a game console, a Global PositioningSystem (GPS) receiver, or a portable storage device, e.g., a universalserial bus (USB) flash drive, to name just a few.

Computer readable media suitable for storing computer programinstructions and data include all forms of non volatile memory, mediaand memory devices, including by way of example semiconductor memorydevices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks,e.g., internal hard disks or removable disks; magneto optical disks; andCD ROM and DVD-ROM disks.

To provide for interaction with a user, embodiments of the subjectmatter described in this specification can be implemented on a computerhaving a display device, e.g., a CRT (cathode ray tube) or LCD (liquidcrystal display) monitor, for displaying information to the user and akeyboard and a pointing device, e.g., a mouse or a trackball, by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide for interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback, e.g.,visual feedback, auditory feedback, or tactile feedback; and input fromthe user can be received in any form, including acoustic, speech, ortactile input. In addition, a computer can interact with a user bysending documents to and receiving documents from a device that is usedby the user; for example, by sending web pages to a web browser on auser's device in response to requests received from the web browser.Also, a computer can interact with a user by sending text messages orother forms of message to a personal device, e.g., a smartphone that isrunning a messaging application, and receiving responsive messages fromthe user in return.

Data processing apparatus for implementing machine learning models canalso include, for example, special-purpose hardware accelerator unitsfor processing common and compute-intensive parts of machine learningtraining or production, i.e., inference, workloads.

Machine learning models can be implemented and deployed using a machinelearning framework, e.g., a TensorFlow framework, a Microsoft CognitiveToolkit framework, an Apache Singa framework, or an Apache MXNetframework.

Embodiments of the subject matter described in this specification can beimplemented in a computing system that includes a back end component,e.g., as a data server, or that includes a middleware component, e.g.,an application server, or that includes a front end component, e.g., aclient computer having a graphical user interface, a web browser, or anapp through which a user can interact with an implementation of thesubject matter described in this specification, or any combination ofone or more such back end, middleware, or front end components. Thecomponents of the system can be interconnected by any form or medium ofdigital data communication, e.g., a communication network. Examples ofcommunication networks include a local area network (LAN) and a widearea network (WAN), e.g., the Internet.

The computing system can include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other. In someembodiments, a server transmits data, e.g., an HTML page, to a userdevice, e.g., for purposes of displaying data to and receiving userinput from a user interacting with the device, which acts as a client.Data generated at the user device, e.g., a result of the userinteraction, can be received at the server from the device.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinvention or on the scope of what may be claimed, but rather asdescriptions of features that may be specific to particular embodimentsof particular inventions. Certain features that are described in thisspecification in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially be claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings and recited inthe claims in a particular order, this should not be understood asrequiring that such operations be performed in the particular ordershown or in sequential order, or that all illustrated operations beperformed, to achieve desirable results. In certain circumstances,multitasking and parallel processing may be advantageous. Moreover, theseparation of various system modules and components in the embodimentsdescribed above should not be understood as requiring such separation inall embodiments, and it should be understood that the described programcomponents and systems can generally be integrated together in a singlesoftware product or packaged into multiple software products.

Particular embodiments of the subject matter have been described. Otherembodiments are within the scope of the following claims. For example,the actions recited in the claims can be performed in a different orderand still achieve desirable results. As one example, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In some cases, multitasking and parallel processing may beadvantageous.

What is claimed is:
 1. A system comprising one or more computers and oneor more storage devices storing instructions that when executed by theone or more computers cause the one or more computers to implement aneural network for generating a network output by processing an inputsequence having a respective network input at each of a plurality ofinput positions in an input order, the neural network comprising: afirst neural network comprising a sequence of one or more subnetworks,each subnetwork configured to (i) receive a respective subnetwork inputfor each preceding input position that precedes a current inputposition, and (ii) generate a respective subnetwork output for eachpreceding input position, and wherein each subnetwork comprises: aself-attention sub-layer that is configured to receive the respectivesubnetwork input for each preceding input position and, for eachparticular input position of the preceding input positions: apply aself-attention mechanism over the subnetwork inputs at the precedinginput positions to generate a respective output for the particular inputposition, wherein applying a self-attention mechanism comprises:determining a query from the subnetwork input at the particular inputposition, determining keys derived from the subnetwork inputs at thepreceding input positions, determining values derived from thesubnetwork inputs at the preceding input positions, and using thedetermined query, keys, and values to generate the respective output forthe particular input position; and a second neural network comprisingone or more neural network layers that is configured to receive thesubnetwork outputs generated by a final subnetwork of the first neuralnetwork and process the subnetwork outputs to generate the networkoutput.
 2. The system of claim 1, wherein: the network output is anoutput sequence having a respective network output at each of aplurality of output positions in an output order; and the first neuralnetwork auto-regressively generates the output sequence, by at each of aplurality of time steps, generating a network output at an outputposition corresponding to the time step conditioned on the subnetworkinputs and network outputs at output positions preceding the outputposition in the output order.
 3. The system of claim 1, wherein thefirst neural network further comprises: an embedding layer configuredto: for each network input at input positions preceding the currentinput position in the input order: map the network input to an embeddedrepresentation of the network input, and combine the embeddedrepresentation of the network input with a positional embedding of thecorresponding input position of the network input in the input order togenerate a combined embedded representation of the network input; andprovide the combined embedded representations of the network input asinput to a first subnetwork in the sequence of subnetworks.
 4. Thesystem of claim 1, wherein at least one of the subnetworks comprises: aposition-wise feed-forward layer that is configured to, for eachparticular input position preceding the current input position: receivean input at the particular input position, and apply a sequence oftransformations to the input at the particular input position togenerate an output for the particular input position.
 5. The system ofclaim 4, wherein the sequence of transformations comprises two learnedlinear transformations separated by an activation function.
 6. Thesystem of claim 4, wherein the at least one subnetwork furthercomprises: a residual connection layer that combines the outputs of theposition-wise feed-forward layer with the inputs to the position-wisefeed-forward layer to generate a residual output, and a layernormalization layer that applies layer normalization to the residualoutput.
 7. The system of claim 1, wherein each subnetwork comprises: aself-attention sub-layer that is configured to: receive an input foreach particular input position preceding the current input position and,for each particular input position: apply an attention mechanism overthe inputs at the particular input positions preceding the current inputposition using one or more queries derived from the input at theparticular input position to generate a updated representation for theparticular input position.
 8. The system of claim 7, wherein eachself-attention sub-layer comprises a plurality of self-attention layers,and wherein each self-attention layer is configured to: apply a learnedquery linear transformation to the input at each particular inputposition preceding the current input position to generate a respectivequery for each particular input position, apply a learned key lineartransformation to each input at each particular input position precedingthe current input position to generate a respective key for eachparticular input position, apply a learned value linear transformationto each input at each particular input position preceding the currentinput position to generate a respective key for each particular inputposition, and for each of the particular input positions preceding thecurrent input position, determine a respective input-position specificweight for each particular input position by applying a comparisonfunction between the query for the particular input position and thekeys, and determine an initial attention output for the particular inputposition by determining a weighted sum of the values weighted by thecorresponding input-position specific weights for the particular inputposition.
 9. The system of claim 8, wherein the self-attention sub-layeris configured to combine the attention outputs generated by theself-attention layers to generate the output for the self-attentionsub-layer.
 10. The system of claim 8, wherein the attention layersoperate in parallel.
 11. The system of claim 7, wherein each subnetworkfurther comprises: a residual connection layer that combines the outputsof the self-attention sub-layer with the inputs to the self-attentionsub-layer to generate a residual output, and a layer normalization layerthat applies layer normalization to the residual output.
 12. One or morenon-transitory computer storage media storing instructions that whenexecuted by one or more computers cause the one or more computers toimplement a neural network for generating a network output by processingan input sequence having a respective network input at each of aplurality of input positions in an input order, the neural networkcomprising: a first neural network comprising a sequence of one or moresubnetworks, each subnetwork configured to (i) receive a respectivesubnetwork input for each preceding input position that precedes acurrent input position, and (ii) generate a respective subnetwork outputfor each preceding input position, and wherein each subnetworkcomprises: a self-attention sub-layer that is configured to receive therespective subnetwork input for each preceding input position and, foreach particular input position of the preceding input positions: apply aself-attention mechanism over the subnetwork inputs at the precedinginput positions to generate a respective output for the particular inputposition, wherein applying a self-attention mechanism comprises:determining a query from the subnetwork input at the particular inputposition, determining keys derived from the subnetwork inputs at thepreceding input positions, determining values derived from thesubnetwork inputs at the preceding input positions, and using thedetermined query, keys, and values to generate the respective output forthe particular input position; and a second neural network comprisingone or more neural network layers that is configured to receive thesubnetwork outputs generated by a final subnetwork of the first neuralnetwork and process the subnetwork outputs to generate the networkoutput.
 13. The non-transitory computer storage media of claim 12,wherein: the network output is an output sequence having a respectivenetwork output at each of a plurality of output positions in an outputorder; and the first neural network auto-regressively generates theoutput sequence, by at each of a plurality of time steps, generating anetwork output at an output position corresponding to the time stepconditioned on the subnetwork inputs and network outputs at outputpositions preceding the output position in the output order.
 14. Thenon-transitory computer storage media of claim 12, wherein the firstneural network further comprises: an embedding layer configured to: foreach network input at input positions preceding the current inputposition in the input order: map the network input to an embeddedrepresentation of the network input, and combine the embeddedrepresentation of the network input with a positional embedding of thecorresponding input position of the network input in the input order togenerate a combined embedded representation of the network input; andprovide the combined embedded representations of the network input asinput to a first subnetwork in the sequence of subnetworks.
 15. Thenon-transitory computer storage media of claim 12, wherein at least oneof the subnetworks comprises: a position-wise feed-forward layer that isconfigured to, for each particular input position preceding the currentinput position: receive an input at the particular input position, andapply a sequence of transformations to the input at the particular inputposition to generate an output for the particular input position. 16.The non-transitory computer storage media of claim 12, wherein eachsubnetwork comprises: a self-attention sub-layer that is configured to:receive an input for each particular input position preceding thecurrent input position and, for each particular input position: apply anattention mechanism over the inputs at the particular input positionspreceding the current input position using one or more queries derivedfrom the input at the particular input position to generate a updatedrepresentation for the particular input position.
 17. A methodcomprising: receiving an input sequence having a respective input ateach of a plurality of input positions in an input order; processing theinput sequence through a first neural network to generate a networkoutput, the first neural network comprising a sequence of one or moresubnetworks, wherein each subnetwork is configured to (i) receive arespective subnetwork input for each preceding input position thatprecedes a current input position, and (ii) generate a respectivesubnetwork output for each preceding input position, and wherein eachsubnetwork comprises: a self-attention sub-layer that is configured toreceive the respective subnetwork input for each preceding inputposition and, for each particular input position of the preceding inputpositions: apply a self-attention mechanism over the subnetwork inputsat the preceding input position to generate a respective output for theparticular input position, wherein applying a self-attention mechanismcomprises: determining a query from the subnetwork input at theparticular input position, determining keys derived from the subnetworkinputs at the preceding input positions, determining values derived fromthe subnetwork inputs at the preceding input positions, and using thedetermined query, keys, and values to generate the respective output forthe particular input position.
 18. The method of claim 17, wherein: thenetwork output is an output sequence having a respective network outputat each of a plurality of output positions in an output order; and thefirst neural network auto-regressively generates the output sequence, byat each of a plurality of time steps, generating a network output at anoutput position corresponding to the time step conditioned on thesubnetwork inputs and network outputs at output positions preceding theoutput position in the output order.
 19. The method of claim 17, whereinthe first neural network further comprises: an embedding layerconfigured to: for each network input at input positions preceding thecurrent input position in the input order: map the network input to anembedded representation of the network input, and combine the embeddedrepresentation of the network input with a positional embedding of thecorresponding input position of the network input in the input order togenerate a combined embedded representation of the network input; andprovide the combined embedded representations of the network input asinput to a first subnetwork in the sequence of subnetworks.
 20. Themethod of claim 17, wherein at least one of the subnetworks comprises: aposition-wise feed-forward layer that is configured to, for eachparticular input position preceding the current input position: receivean input at the particular input position, and apply a sequence oftransformations to the input at the particular input position togenerate an output for the particular input position.